The use of heat pipe based wrap-around heat exchangers (HP-WAHX) is beneficial in regions that have high humidity for most of the year. HP-WAHXs are used to change the performance of the active cooling coils (such as chilled water coiling cools or DX cooling coils). In air conditioning and dehumidification applications, particularly in applications where, for ventilation and indoor air quality purposes, the amount of outside air is relatively high (i.e. 30% or more) the cooling coil must lower the temperature of the air and also condense out excess moisture. Reducing the temperature of the air is called sensible cooling and condensing moisture is called latent cooling. In order to accomplish the level of cooling needed, heat pipe based wrap-around heat exchangers often use large arrays of heat pipes (up to 600 per system) that span two fin coil structures and wrap around existing HVAC components. The heat pipes are generally made of copper with a refrigerant working fluid contained within each sealed pipe. In such systems, it is critical that all parts of the heat pipe envelope remain leak tight.
Variable Conductance Heat Pipes (VCHPs) have been used to vary heat pipe effectiveness (performance) as a function of operating temperature; however, this technology has typically been used to increase power transferred with increasing temperature.
In order for the heat pipes in a WAHX system to operate properly and efficiently, valves may be provided in the heat pipes to control the flow of the working fluid in the heat pipes. When a valve is used, the valve must be hermetically sealed from the outside environment in order for the heat pipe system to function. The valve must also have an open, low pressure drop flow path when it is in the open position.
Currently, known valves are generally electrically controlled valves (solenoid valves and actuated ball valves) which require input electrical or pneumatic power and thermal feedback (temperature sensor and controller). They are also relatively expensive. Other valves, such as phase change material (PCM) valves are known but do not have an open, low pressure drop flow path during operation and are not designed for refrigerant, heat pipe service.
It would, therefore, be beneficial to provide a valve for use in heat pipes which is hermetically sealed, has an open cross section, is compact and is passively thermally actuated. In addition, it would be beneficial to provide such heat pipes which require decreasing power with increasing temperature. In particular, it would be beneficial to provide a valve which uses a phase change material actuator to block off flow through a tube at a predetermined temperature, whereby the valve is fully passive because it is controlled by the expansion and contraction of a phase change material.